Here are a couple of great Preview Handlers for PDF files.
Another one is in this MSDN article. The code download for this article includes a framework that makes it a snap to implement your own preview handlers, and it provides several sample previewers (including previewers for PDF, XML, ZIP, MSI, BIN, CSV, XPS, and XAML files).
If I had more time I would write one for DWG and DWF files. With the above code samples it's a quite easy task.